mode hopping (original) (raw)

Author: the photonics expert (RP)

Definition: the phenomenon that a laser exhibits sudden jumps of optical frequency, which are associated with transitions between different modes of its resonator

Categories: article belongs to category optical resonators optical resonators, article belongs to category laser devices and laser physics laser devices and laser physics

Related: resonator modessingle-frequency laserswavelength tuninglaser resonatorslaser noise

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DOI: 10.61835/32v Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Mode Hopping?

Mode hopping is a phenomenon which is mostly discussed in the context of single-frequency lasers. Under some external influence, such a laser may operate on a single resonator mode for some time, but then suddenly switch to some other mode with a different optical frequency. This means that this other mode suddenly takes over all the optical power; for a short while, there may be significant power in both modes.

In multimode operation of a laser, there may also be transitions between different sets of modes. However, simultaneous oscillation on many modes (often with substantial fluctuations) is then most common, and instead of complete mode hops there are often more continuous transitions, with the optical power being gradually redistributed. The dynamics can be further influenced by nonlinear optical effects such as spatial hole burning.

Mode hops can also involve higher-order modes, or modes with different polarization in lasers with polarization-independent gain.

Causes of Mode Hops

Mode hops are often provoked by internal and external influences. For example, a drift of the temperature of the gain medium will shift the wavelength of maximum gain while not shifting the frequencies of the resonator modes to the same extent. (This can happen e.g. in laser diodes, where temperature changes usually affect the gain maximum more than the cavity resonances.) The previously lasing mode may then no longer be the mode with highest gain, so that the power of a competing mode with higher gain can quickly rise.

Essentially the same phenomenon can occur for length drifts of the laser resonator, which shift the resonator mode frequencies without also shifting the gain maximum.

Both origins of mode hops often result from attempts to tune the wavelength of a laser. The external influences can of course also be just random noise, e.g. mirror vibrations, temperature fluctuations or changes in pump power, or external optical feedback.

Consequences of Mode Hops

Mode hops can obviously be disturbing because they can make it difficult to obtain continuous wavelength tuning, for example. Depending on various details of a laser setup, the wavelength may make significant jumps with only small changes between these jumps, or it can vary like a sawtooth curve.

Mode hops also generate laser noise, e.g. in the form of intensity noise and noise of other parameters, particularly if the mode hops occur frequently.

Avoiding Mode Hopping

For single-frequency lasers, it is often desirable to avoid mode hopping, achieving a stable single-frequency emission.

It is generally easier to avoid mode hopping when the laser resonator has a large free spectral range, i.e. a large frequency spacing of its fundamental modes. Some tunable lasers are just equipped with a correspondingly short laser resonator, and the tuning range is limited by a value close to the free spectral range, which may e.g. be a few gigahertz for a compact nonplanar ring oscillator (NPRO or MISER).

A continuous (mode-hop-free) tuning range much wider than the free spectral range is possible by coordinated tuning of the resonator mode frequencies and the gain maximum, or with an additional intracavity optical filter.

Random mode hops may be suppressed by minimizing external noise influences (e.g. by temperature stabilization) and also by using a resonator with large free spectral range (see above).

It is also possible to employ various kinds of nonlinearities which favor the already lasing mode over competing modes. An example is spatial hole burning in an unpumped region of a quasi-three-level laser gain medium [3]. A similar effect can be achieved with intracavity frequency doubling [2].

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is laser mode hopping?

Mode hopping is a phenomenon where a single-frequency laser, after operating on a single resonator mode, abruptly switches to another mode with a different optical frequency. The new mode then suddenly takes over all the optical power.

What are the main causes of mode hopping?

Mode hopping is often caused by drifts in the temperature of the gain medium or in the length of the laser resonator. These drifts shift the wavelength of maximum gain relative to the resonator mode frequencies, allowing a competing mode to achieve higher gain and begin to lase.

What are the consequences of mode hopping?

Mode hops are problematic as they prevent continuous wavelength tuning, causing sudden jumps in wavelength. They also generate laser noise, such as intensity noise, especially when they occur frequently.

How can mode hopping be avoided in a laser?

Mode hopping can be suppressed by using a laser resonator with a large free spectral range, which increases the frequency spacing between modes. Additionally, stabilizing the laser's temperature and coordinating the tuning of the resonator modes and gain peak can achieve continuous, mode-hop-free operation.

Does mode hopping also happen in multimode lasers?

In multimode lasers, complete mode hops are less common. Instead, power is often gradually redistributed between different sets of modes. These dynamics can be complex and influenced by effects like spatial hole burning.

Bibliography

[1] G. A. Ball and W. W. Morey, “Continuously tunable single-mode erbium fiber laser”, Opt. Lett. 17 (6), 420 (1992); doi:10.1364/OL.17.000420
[2] K. I. Martin et al., “Self-suppression of axial mode hopping by intracavity second-harmonic generation”, Opt. Lett. 22 (6), 375 (1997); doi:10.1364/OL.22.000375
[3] R. Paschotta et al., “Single-frequency ytterbium-doped fiber laser stabilized by spatial hole burning”, Opt. Lett. 22 (1), 40 (1997); doi:10.1364/OL.22.000040
[4] C. Petridis et al., “Mode-hop-free tuning over 80 GHz of an extended cavity diode laser without antireflection coating”, Rev. Sci. Instrum. 72 (10), 3812 (2001); doi:10.1063/1.1405783

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